Certain strains of E. coli causes extraintestinal disease, ranging from cystitis to sepsis. Complement resistance appears to be a vital trait in the pathogenesis of infections by these strains. An improved understanding of the complement resistance determinants in this group of E. coli may to a logical development of new preventative and/or therapeutic strategies. The experimental approach taken for this project was to study a serum resistant, non-K1, blood isolate as a model pathogen; as these serotypes are responsible for the majority of adult disease caused by extra- intestinal isolates of E.coli. Transposon mutagenesis (TnphoA) was utilized to create a library of mutants in outer, periplasmic and cytoplasmic membrane proteins. Mutants were screened for an increased sensitivity to serum. Isogenic group 2 (K54 antigen) capsule deficient mutants were identified with an increased sensitivity to serum, undergoing a 3-4 log-kill over 3 hours in 80% serum. Further in vitro evaluations established that the K54 capsular polysaccharide protects against killing by the alternative complement pathway. In addition, C3 binding was equivalent in both strains. Therefore, the mechanism of complement resistance conferred by the K54 capsule is different than that of K1. The E. coli LPS has been purported to be the major complement resistance determinant. Therefore, we undertook and were successful in creating proven isogenic LPS-minus (O specific chain) derivatives of CP9 as well as isogenic CP9 derivatives deficient in both the group 2 capsule (K54) and the 0 specific chain of the LPS (04). The results of our evaluations of these mutants, however, are in contrast to those of previous reports. The 04 antigen appears to play only a minor role in protecting against complement mediated killing. However, the loss of both the K54 antigen and the 04 render the strain extremely serum sensitive.The results of these studies expands on the understanding of complement resistance mechanisms in E. coli. This project is being terminated at N.I.H.